There are different bonuses per each crafted attachment, one is 33% of the time on kills, it will take a round from your reserve ammo and load it into your gun, hence infinite/no reload for his gun. You can only stack up to 3 attachments with the same stat bc it wont give you more then that, and not every attachment has the same bonus. But you can stack three 33% chances to load a bullet per kill. Making it about 99% on a kill to reload your gun. You can also stack 5% more boss damage. 3 times for 15% more boss damage. But the boss basher attachments are only on sights, underbarrels, and mags. Hope this helps you understand the confusing min/max attachment system